1. Field of the Invention
Embodiments of the present invention generally relate methods and apparatus for drilling a well beneath water. More specifically, embodiments of the present invention relate to methods and apparatus for drilling a deep water well.
2. Description of the Related Art
In well completion operations, a wellbore is formed to access hydrocarbon-bearing formations by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill support member, commonly known as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annular area is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annular area with cement. The casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole using apparatuses known in the art. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with a drill bit on a drill string. The drill string is removed. A first string of casing or conductor pipe is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string may then be fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to frictionally affix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
In the construction of deep water wells, a conductor pipe is typically installed in the earth prior to the placement of other tubulars. Referring to FIG. 1, the conductor pipe 10, typically having a 36″ or 30″ outer diameter (“OD”), is jetted, drilled, or a combination of jetted & drilled into place. Placement depth of the conductor pipe 10 may be approximately any where from 200 to 500 feet below the mud line 7. As shown in FIG. 1, the conductor pipe 10 is typically carried in from a drill platform 3 on a drill string 12 that has a bit or jetting head 15 projecting just below the bottom of the conductor pipe 10, which is commonly referred to as a bottom hole assembly (“BHA”). The conductor pipe 10 is placed in the earth by jetting a hole and if necessary partially drilling and/or jetting a hole while simultaneously carrying the conductor pipe 10 in. A mud motor 18 is optionally used above the jetting/drilling bit 15 to rotate the bit 15. The conductor pipe 10 is connected to the drill string 12 with a latch 20. See also FIG. 2. Typically a drill string latch 20 fits into a profile collar 22 built into the conductor pipe 10. Once the conductor pipe 10 is jetted and/or drilled to the target depth, a ball is dropped through the drill string 12 from the surface. The ball provides a temporary shut off of the drill string 12 to allow pressurization of the drill string 12 in order to hydraulically release the latch 20 from the conductor pipe 10. (The latch can also be released by pipe manipulation, and not require the dropping of a ball.) Thereafter, fluid flow through the drill string 12 is re-established so that the drill string 12 can drill ahead to create a hole for the next string of casing.
The general procedure for drilling the hole below the conductor pipe to install the structural or surface casing is to drill with a BHA on the end of the drill string used to run the conductor pipe in the hole. Surface casing is casing run deep enough to cover most know shallow drilling hazards, yet the casing is typically located above any anticipated commercial hydrocarbon deposits. The BHA will as a minimum consist of a drilling or jetting bit. The BHA may also contain a mud motor, instrumentation for making geophysical measurements, an under reamer, stabilizers, as well as a drill bit or an expandable drill bit.
The hole is normally drilled with sea water or an environmentally friendly drilling fluid, which is also known as “mud”. Sea water or environmentally friendly mud is used because the drilling fluid is allowed to exit into open water at the top of the conductor pipe. This drilling method is generally referred to as riserless drilling (also referred to as the “pump and dump” drilling method). The reason this method is used is because the riser, which is a pipe run from the top of the well at the mud line to the rig, has to be supported at the mud line. In the earlier stages of casing placement, support for the riser is often unavailable. If a riser is in place, the drill string is run inside the riser, thereby forming an annulus between the OD of the drill string and the inside diameter (“ID”) of the riser. The annulus provides a path for the drilling fluid to return to the rig during the drilling process. To get the support required to run the riser, the structural casing and/or the surface casing must be in place first.
The surface casing hole is typically drilled to a target depth and then a viscous “pill” made up of weighted and/or thickened fluid is placed in the hole as the drill string is extracted from the hole. The viscous pill is intended to keep any formation or ocean flows from flowing into the drilled hole and to keep the hole from collapsing before the casing is run in the hole. Another purpose of the viscous pill is to keep cement from filling up the rat hole after the surface casing is placed and while it is being cemented in. The rat hole is the difference in depth between the bottom of the casing and the bottom of the hole and is created by drilling deeper than the length of the casing to be run. If cement fills the rat hole, then the next drill string that goes through the cement in the rat hole may core it and the remaining cement, since it is unsupported could fracture and fall in on the drill string, thereby possibly trapping the drill string in the hole.
In some instances, a weighted fluid such as a drilling mud or weighted brine is required to control formation flows of a shallow water flow and/or a shallow gas flow. As an example, if the hole is being drilled at 90 feet per hour and the target depth is 2000 feet, it will take in excess of 22 hours to drill the well, and if the pump rate is 900 gallons per minute during drilling, it will take approximately 1,200,000 gals of weighted fluid to drill the well. Because this occurs during the riserless stage, most of the weighted fluid will be lost to the open water. Due to the cost of weighted fluids, many operators provide the BHA with instrumentation to determine when to switch from sea water to weighted fluid. The primary instrument used is the Pressure While Drilling or “PWD”. The PWD will monitor annular pressure to detect a change in pressure that could indicate the drill bit has penetrated a shallow water or gas flow. When that occurs, the drilling fluid is weighted up and pumped down the drill string to the bit. However, for the fluid to be effective in shutting off the flow, enough weighted fluid must be supplied to fill the hole to a level above the bit for the fluid to have enough hydrostatic head to stop the flow. For a 26″ ID hole with an 8″ OD drill string 25 gallons of fluid per foot is needed to fill the hole. Even with the assistance of PWD, a significant amount of weighted drilling fluid must still be used.
With the conductor pipe at the target depth and the latch released, and the hole drilled for the next casing string the drill string is pulled out of the hole (“POOH”) back to the rig floor and the conductor pipe stays in the hole. The conductor pipe is typically not cemented in place.
With the conductor pipe in place and the hole drilled for the next string of casing, the next step may be to install structural pipe or surface casing. Some wells may require structural pipe ahead of the surface casing. The structural pipe is typically placed in a well to help mitigate a known drilling hazard(s), e.g., shallow water flow, shallow gas flow, and low pore pressure. Wells with these types of drilling hazards tend to fracture when the minimum drilling fluid weight needed to control shallow water flows and/or shallow gas flows is used. Structural pipe may also help support the wellhead.
Running large diameter casing in a predrilled hole presents several challenges. One such challenge arises when the hole has low formation pore pressure. In that instance, running the casing too fast could surge the well, i.e., put excessive pressure on the bore of the well, and cause the bore hole to fracture or break down a surrounding earth formation. Typically, breaking down or fracturing the formation causes the formation to absorb fluid. The normal method of keeping the surge pressures low is to run the casing slowly. On drilling rigs, the extra time needed to run the casing may substantially increase the operating cost.
A need, therefore, exists for apparatus and methods of running casing into the earth below water. There is also a need to quickly drill and case a well, preferably in a single trip.